1. Field of the Invention
The present invention relates to a method and to a device intended for seismic monitoring of an underground zone such as a reservoir, comprising simultaneously using seismic vibrators.
2. Description of the Prior Art
It is well-known to monitor the long-term state variations of a reservoir during production, such as a hydrocarbon reservoir or a reservoir intended for gas storage, by means of a seismic system comprising an impulsive seismic source or a seismic vibrator emitting seismic waves in the ground and a reception device comprising seismic pickups arranged at the surface or in wells and coupled with the formations to be monitored. At predetermined time intervals, seismic investigations are carried out with wave emission, the waves reflected by the subsoil discontinuities are received and seismograms are recorded, so as to determine by comparison the changes that take place in the reservoir as a result of the development thereof.
Various long-term seismic monitoring systems are described for example in patents U.S. Pat. Nos. 5,461,594, 4,775,009, and 5,724,311 and French Patent 2,775,349.
U.S. Pat. No. 5,724,311 and French Patent 2,775,349 notably describe systems intended for seismic monitoring of an underground zone during development which may be for example a hydrocarbon reservoir or a gas storage reservoir. As diagrammatically shown in FIGS. 1 to 3, these systems comprise for example a network of seismic antennas 2 consisting each of a series of seismic pickups 4 arranged at regular intervals along a well 3 drilled in the ground. This network can be regular as shown in FIG. 2, or irregular. The pickups can be one-directional geophones oriented vertically or multi-axis geophones (triphones) and/or hydrophones. A seismic source 5 is arranged in the vicinity of each antenna 2. Piezoelectric type vibrators, such as those described in French patent application 99/04,001 are advantageously used as sources and permanently installed in the immediate neighbourhood of each antenna 2.
The seismic waves generated by the or by each seismic source 5 are propagated downwards (downgoing waves 9). These incident waves are first recorded by receivers 4 in each well 3. The waves reflected by the discontinuities of the zone (seismic interfaces) are propagated upwards. These upgoing waves 10 are also recorded by the various receivers 4. The upgoing waves and the downgoing waves are thus superimposed on the seismograms. They are usually processed by means of a method similar to the VSP (Vertical Seismic Profiles) processing method which is well-known to those skilled in the art.
The various sources of the seismic system can be actuated successively by providing, between each triggering, a sufficient time interval for reception of the waves reflected by the investigated zone. Several seismic sources emitting the;same signals can also be used and triggered simultaneously in order to increase the power emitted.
U.S. Pat. No. 4,780,856 also describes a marine seismic prospecting method comprising emission of seismic waves by a vibrator or simultaneously by several vibrators controlled by coded vibrational signals according to a pseudo-random code.
The method according to the invention allows seismic monitoring of an underground formation. It comprises:
emission of seismic waves in the formation by coupling with the formation at least two vibrators which seismic waves simultaneously and are controlled by orthogonal signals so as to form a composite vibrational signal,
reception of the signals reflected by the formation in response to the emission of seismic waves,
recording the signals received by at least one seismic pickup, and
formation of seismograms by processing the signals recorded, comprising discrimination of the respective contributions of the vibrators to the composite vibrational signal and reconstruction of seismograms equivalent to those that would be obtained by actuating the vibrators separately.
Sinusoidal signals of different frequencies, in their fundamental components as well as in their respective harmonics, or signals based on wavelets, on Legendre polynomials or on random series, etc, are for example used as orthogonal signals.
In the case of the emitted orthogonal signals being sinusoids, discrimination of the respective contributions of the vibrators is for example carried out by determining the amplitude and the phase of the composite vibrational signal at the fundamental frequencies of the pilot signals applied to the vibrators.
Discrimination of the respective contributions of the vibrators comprises for example weighting the recorded signal by a bell weighting (or tapering) factor and determining the amplitude and the phase of the composite signal.
In order to carry out discrimination of the respective contributions of the vibrators, a selection by Fourier transform of the lines of the complex spectrum respectively associated with the various weighted signals is for example performed.
Reconstruction of the seismograms, specifically corresponding to the various vibrators, is performed for example by applying, after separation thereof, an inverse Fourier transform to the lines respectively associated with the various weighted signals.
According to an implementation mode, the frequencies of the orthogonal pilot signals respectively applied to the various vibrators are shifted by frequency intervals, at predetermined time intervals, so as to sweep a certain emission frequency band.
The system of seismic monitoring of an underground formation according to the invention comprises means allowing emission of seismic vibrations in the formation comprising at least two vibrators and means for generating orthogonal signals and for applying the orthogonal signals respectively to the vibrators so as to generate in the formation a composite vibrational signal, means for receiving the signals reflected by the formation in response to the emission of seismic waves, means for recording the signals received by the means for receiving and means for processing the signals recorded in order to form seismograms, comprising at least one computer which discriminates respective contributions of the vibrators to the composite vibrational signal and reconstructs seismograms equivalent to those that would be obtained by separately actuating the vibrators.
According to a first implementation mode, the system comprises at least two local units arranged at a distance from one another and coupled with the formation. Each unit comprises at least one seismic pickup, a seismic vibrator, a local device which acquires and processes the received signals , and a central control and synchronization unit connected to the various units, comprising a generator which applies orthogonal vibrator pilot signal to the vibrators.
According to another implementation mode, the system comprises at least two local units arranged at a distance from one another and coupled with the formation. Each unit comprises at least one seismic pickup, a seismic vibrator, and a central control and synchronization unit connected to the various local units by a coupling mechanism (cables for example) or a wireless link (radio) and comprising a signal generator which forms the orthogonal vibrational pilot signals, and means which acquires signals received by antennas and which reconstructs seismograms corresponding to the contributions of the vibrators.
The reception means comprise, for example, at least one antenna comprising seismic pickups arranged along a well drilled in the formation and which is connected to the recording means.